The invention described herein may be manufactured, used, and licensed by or for the United States Government for governmental purposes without the payment to us of any royalty thereon.
In the 1969 to 1975 time frame, the US Army had two Research and Development (RandD) efforts aimed at developing printed circuit Traveling Wave Tubes (TWTs). One effort utilized a meanderline as the slow-wave printed circuit on a dielectric substrate and a sheet electron beam to obtain amplification. The second effort utilized an equiangular spiral slow-wave printed circuit on a dielectric substrate and a radial traveling electron beam to obtain amplification. The primary goal of both RandD efforts was to demonstrate the feasibility of a TWT that was lower cost than a conventional TWT, and bridged the gap between solid state technology and vacuum technology for microwave oscillator/amplifier devices. The low cost of the TWT was achieved by printing on a pair of Ceramic substrates all of the internal tube parts except the cathode-grid assembly and spacers required to have a vacuum gap for beam flow. That is, the beam forming electrodes, collector, and microwave and electric connections are printed on a pair of ceramic substrates, which have two identical printed microwave slow-wave circuits. Amplification of a microwave signal propagating on the slow-wave circuits occurs by the well-known beam-wave, circuit-wave interaction. The amplification mechanism requires velocity synchronism between the space-charge wave on the beam and the electromagnetic (EM) wave on the circuit, where dc energy is extracted from the beam and converted to microwave energy. The electron beam is generated by a thermionic cathode (heated cathode) or field-emitter cathode (cold cathode), focused by beam forming electrodes (grid/anode) and magnet structure, and collected by the printed collector. For the equiangular spiral TWT, the sheet beam is a radial directed beam that travels outward from the cathode located on an innermost circumference to the collector located on an outermost circumference. The linear beam TWTs were designed and built to operate in S-band and the radial beam TWT was designed and built to operate in L hand from 0.5-1.5 GHZ. A C-band, linear beam TWT was designed and it is described in xe2x80x9cA Design Study of C-band Printed Circuit TWTxe2x80x9d an Army report dated May 1971.
Some technical problems were not solved in the 1970""s, which adversely affected tube performance and thus were obstacles in achieving prototype production tubes. The ceramic substrates have a large dielectric loading effect, which lowered the interaction impedance, gain, and efficiency. Partial solutions to these problems compromised high-duty cycle operation. In order to achieve a higher gain and efficiency, air or low dielectric material gaps were placed between the ceramic substrates and metal tube housing. The gaps reduced the energy stored between the ceramic substrates and metal tube housing. This improved the beam interaction, gain, and efficiency at the expense of duty cycle, since the air gaps made it more difficult to transfer heat generated inside the tube to the outside environment. Also, the air gaps caused a more rapid gain roll-off over the frequency bandwidth of operation.
This invention replaces the ceramic substrates and metal ground planes with Photonic Band Gap (PBG) crystal structures. In particular the two- or three-dimensional Metallodielectric Photonic Crystals (MPCs) are used as the supporting structures for the printed slow-wave interaction circuits. This will significantly increase the interaction impedance, gain, and efficiency without compromising gain roll-off and duty cycle. The air or low dielectric material gaps are not needed between the PBG structures and tube housing to significantly improve the interaction impedance.
The two-dimensional MPCs (high-impedance surfaces) have surface band gaps that reduce EM propagation (typically xe2x88x9210 to xe2x88x9220 dB) through the crystal. They also forbid surface currents, unlike metals. The three-dimensional MPCs can be made to have both bulk and surface band gaps, and these two band gaps can be engineered to overlap. The bulk band gap forbids EM propagation (typically xe2x88x9240 to xe2x88x9260 dB) through the crystal. They also forbid surface currents, unlike metals. In addition, the MPCs are excellent heat sinks because they contain metal elements. In particular, the metal elements of the two-dimensional MPCs are attached to the ground plane. The excellent heat sink property allows high duty operation of the TWTs.
Accordingly, it is an object of the present invention to provide a TWT that is compact with a low-cost design.
It is another purpose of this invention to improve tube performance over prior art printed circuit TWTs.
It is also another purpose to reduce or eliminate oscillations in the TWT.
Another objective of this invention is to eliminate the air or low-dielectric material gaps between the substrates and tube housing that were used in prior art.
A further objective of this invention is to increase the critical frequency of printed circuit TWTs.
Another objective of this invention is to have multiple devices in one package.
The foregoing and other objects are achieved by an invention in which all of the tube""s internal parts are printed on metallodielectric photonic crystal (MPC) structures except for the cathode-grid assembly and spacers required to maintain a vacuum gap for the electron beam propagation region.
This invention has higher duty cycle capability, higher interaction impedance, larger bandwidth, and higher critical frequency over prior art, which in turn gives higher gain, higher rated power and higher efficiency of the TWT.
These objectives are realized by using PBG crystals with one or more defects as the structures for the printed slow-wave interaction circuits. It is well known by tube designers that the radio frequency (RF)/microwave signal when coupled onto a slow-wave circuit decays approximately exponentially away from the circuit. If the circuit is on a dielectric substrate, dielectric loading further decreases the EM fields in the vicinity of the electron beam. It is highly desirable to have large EM fields in the direct vicinity of the electron beam to significantly increase the interaction impedance, gain, and efficiency. The PBG crystal accomplishes this because it is designed to have a forbidden band gap over the bandwidth that the TWT is designed to operate. An incoming EM signal whose carrier frequency is well within the forbidden band gap, and whose line width is finite, cannot penetrate (usually at least xe2x88x9220 dB) the crystal, and is reflected away from the crystal. For a PBG crystal composed of low-loss media, (loss-tangent  less than  less than 1), large electric field oscillations, are built up in the direct vicinity of the beam, which causes the beam to bunch. Coupling of the beam with the EM circuit wave occurs when the beam velocity slightly exceeds the phase velocity of the circuit mode. Forward and backward operation of the TWT is possible. When the phase and group velocities are in the same direction, forward operation occurs. When the phase and group velocity are in the opposite direction, backward operation occurs. Operation in the forward mode gives higher power (amplification) and larger instantaneous bandwidth; operation in the backward mode gives voltage tunability.
The interaction impedance is furthered enhanced because the beam is sandwiched between two PBG crystal structures. The EM fields that decay away from the circuit on one PBG crystal structure in the direction of the other PBG crystal structure are also forbidden from entering that substrate which causes high EM fields to build up in direct vicinity of the electron beam.
Suppression of internal oscillations can be a serious problem, especially, for high-gain tubes. Techniques are needed to prevent high EM fields from existing in unwanted modes. PBG crystals that have induced defects can reduce or eliminate oscillations. The perfect two or three-dimensional translational symmetry of a PBG crystal can be lifted in either one of two ways: (1) extra dielectric (permittivity), or permeability, or metal materials can be added to one or more of the unit cells. This type defect behaves much like a donor atom in a semiconductor that gives rise to donor modes with origins at the bottom of the conduction band. (2) Conversely, by removing some dielectric/permeability material from one or more of the unit cells, defects occur which resemble acceptor atoms in semiconductors. The PBG crystal can be designed to have donor and acceptor defects, which allow EM transmission (pass bands) through the PBG crystal at frequencies which are functions of the defects. Therefore, to prevent oscillation buildup at a given frequency, one can create a defect(s) in the PBG crystal at the oscillation frequency thus reducing the EM fields in the vicinity of the beam. The acceptor/donor level frequency within the forbidden band gap is a function of the defect volume removed or added. The technique of creating defects in PBG crystals to prevent oscillations is a significant improvement over conventional techniques such as cutting slits in the circuit or adding distributed loss on the circuit by painting a lossy material such as aquadag. These techniques can increase the insertion loss by greater than 10 dB which means the circuit length has to be extended to obtain reasonable gain.
Another objective of this invention is to eliminate the air or low-dielectric material gaps between the substrates and tube housing that were used in prior art. The gaps were found to be necessary to raise the interaction impedance in the prior art printed circuit TWTs. However, the gaps lower the duty cycle because it is difficult to transfer heat buildup inside the tube to the outside environment. In addition, the gain response of the tube with the gaps was shown in prior art to falloff more rapidly, which narrows the bandwidth. In the prior art, the region between the outer (back) surfaces of the ceramic substrates and tube housing stored energy due to fringing fields. By removing the ground plane away from the back surfaces of the ceramic substrates and creating an air or low dielectric material gap, the interaction impedance increases and the useful bandwidth decreases because the circuit becomes more dispersive. In this invention, the MPC structures do not allow surface currents. Thus FM energy with frequency content in the forbidden band gap can not leak behind the structures, and can not effectively penetrate the PBG structures due to the band gap. Since the air or low-dielectric gap is not required for this invention, the interaction impedance, bandwidth, and duty cycle are improved over prior art printed circuit TWTs. The duty cycle is further enhanced by this invention since heat generated on the slow-wave circuits can be conducted to the outside environment via the metal elements in the MPCs and ground planes. The two-dimensional MPC is an excellent heat sink, since it is a thin structure with the metal elements inside the MPC attached directly to a ground plane, which in-turn is in direct contact with the metal vacuum housing.
A further objective of this invention is to increase the critical frequency of printed circuit TWTs. This frequency is where rapid fall-off of gain occurs. It was found that the critical frequency, fc for the equiangular spiral amplifier is proportional to 1/(∈r+1). That is fcxe2x88x9d1/(∈r+1). As an example, for a dielectric substrate with a dielectric constant of 8, fc is reduced by a factor of 3. For a two- or three-dimensional, 50-ohm impedance MPC structure, the EM fields penetrating the structure are drastically reduced, and are reflected at its surface when the frequency content lies within the PBG. Therefore, the effective dielectric constant that the microwave signal sees approaches a value of 1. The sheet insulator that supports the slow-wave circuit (see FIG. 5) can have a low dielectric constant of less than 4, and since this insulator sheet is very thin ( less than  less than  less than xcex0), its effective dielectric constant is negligible. Therefore, fc is only reduced by a factor of about 3. Thus the critical frequency of the TWT would be about 1.7 times higher for the 50-ohm PBG crystal structure as compared to the dielectric substrate used in prior art. This means that the TWT can be designed to have a bandwidth that is as much as 50% higher than the prior art printed circuit TWTs.
Other examples of how the dielectric constant of the ceramic substrate adversely affects tube performance for the planar equiangular spiral amplifier are:
Interaction impedance, Kxe2x88x9d1/(∈r+1),
gain, Gxe2x88x9d1/(∈r+1)⅓,
phase velocity, vpxe2x88x9d1/(∈r+1), and
maximum power output at any frequency, Poxe2x88x9d1/(∈r+1)321. Lowering the effective dielectric constant, ∈r of the substrates that support the slow-wave circuits is highly beneficial to achieve higher efficiency.
Another objective of this invention is to have multiple devices in one package. For example, the oscillator driver that is needed to excite a TWT amplifier, can be printed on the same PBG structure. Since surface currents are eliminated with the MPC structures, multiple devices are electrically isolated at high radio frequencies (RF) with no cross talk or microwave coupling.
In one embodiment, the printed circuit TWT is composed of two identical PBG crystal structures with two identical meanderline slow-wave circuits printed on them, arranged in a parallel fashion with a vacuum gap (for beam flow) between them, and placed in a housing which forms a vacuum envelope. All of the internal elements are printed on the inner surfaces of the PBG crystal structures except the gridded electron gun and spacers, which are the only non-printed elements inside the tube. The magnet focusing structure is placed on the outer surface of the tube housing. The electrical and RF input and output connections are brought into the tube via standard connectors and are connected internally by printed coupling lines when required such as for the one or two stage printed depressed collector. Some design features of this embodiment are given below. Design features may vary from tubes built for specific applications and those design changes are well known to people skilled in the art.
1. Multiple strapped meanderline slow-wave circuits
2. Period tapering of the meanderline to improve synchronism between beam and wave
3. Non-intercepting or intercepting gridded gun.
4. Thermionic or field emitter cathode
5. Single-stage or multi-stage depressed collector.
6. Air or liquid cooling means.
7. Temperature compensated PPM focusing magnets.
8. Electrical focusing elements for the sheet electron beam.
9. Two-dimensional and three dimensional 50-ohm impedance MPC structures with narrow, wide, or ultra wide forbidden band gaps.
10. PBG crystal defects to reduce or eliminate oscillations, or to change the circuit dispersion characteristics
11. Low-loss tangent and high-voltage breakdown dielectric material for the PBG crystal structures.
12. Ferroelectric PBG material for changing the dispersion characteristics of the MPC and the slow-wave interaction circuit in real time.
13. Two-dimensional and three-dimensional 50-xcexa9 MPC structures to forbid surface currents.
14. Backward or forward wave interaction RF circuits (amplifier or oscillator designs)
15. Space-charge wave or transverse wave beam interactions.
In another embodiment, the printed circuit TWT is composed of two identical PBG crystal structures with two identical equiangular, slow-wave circuits printed on them, arranged in a parallel fashion with a vacuum gap between them, and placed in a housing to form a vacuum envelope. All the elements necessary for generation of microwave power are printed on the inner surfaces of the PBG crystal structures except for the gridded electron gun assembly and spacers. The magnet focusing structure is placed on the outer surface of the housing. The electrical and RF input and output connections are brought into the tube via standard connectors and are connected internally by printed coupling lines when required such as to the printed collector. The printed design techniques of this embodiment are similar to those given above for the first embodiment. Also design features for this embodiment may vary for the tubes built for specific applications, and these design features and changes are well known to people skilled in the art. For example, the two-arm spiral slow-wave circuits (one on each PBG crystal structure) may be wound in the same or opposite sense (clockwise or counter-clockwise).
When one spiral is wound counter-clockwise and the other spiral clockwise, the interaction impedance increases but at compromise of bandwidth. Unlike the meanderline circuit, which has a 10-15 bandwidth, the equiangular spiral circuit exhibits ultra-wide band (multi-octave). The equiangular spiral could be replaced by an Archimedean spiral, which would decrease the bandwidth, but increase the interaction impedance. For this embodiment, a two-arm spiral circuit is used. For higher voltage operation, a four-arm spiral could be utilized with the complication of coupling and uncoupling the RF energy. Higher voltage operation results when more arms are added to the spiral circuit because the spiral arms are not as tightly wound. For this embodiment which, requires multiple input and output connectors, an optoelectronics technique can be used that uses light activated semiconductor switches in conjunction with a mode-locked laser to generate picosecond risetime current pulses. The laser beam, which is jitter-free can be used to switch impulse currents onto the spiral arms which will produce an ultra-wide band microwave signal, for amplification. This technique eliminates the RF input connectors.